British light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight ten-round fight at the Manchester venue.
But the challenger had his moments, delivering effective single shots in a match where the approaches failed to mesh, with excessive holding spoiling the fight.
The scoring judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They succeed," the victor stated in the arena as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th pro success.
Several boxing experts believed Parker, thirty-one, deserved the decision but the Derbyshire fighter suffered the second loss of his 28-match career.
The Champion Earns Victory in Slow Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers prepared to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the more effective rounds.
The spectators' acceptance diminished during regular grabbing, with the challenger slipping to the canvas various instances during clinches.
Parker finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Arthur overcame Bradley Rea by close verdict in a terrific local rivalry battle to secure the European light heavyweight championship.
The arena grew hushed for a deeply poignant homage before the fight as 10 bells rang in memory of Manchester's own Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell ended, the stillness transitioned to a roar of "there's only one Ricky Hatton".
The fighting during the bout matched the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the face in the early round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was marked up soon, noticeable damage appearing under his face as the pair exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor edged the verdict with official scores of even score, 115-113 and clearer margin, but each competitor were persistent and brave, all the qualities their inspiration the legend embodied.
